1 definition
by
stephen jolly is still a very naughty boy

to laugh at your own joke whilst you're still attempting to tell it, resulting in a mumbly laugh (or lumble) escaping your lips.

this usually happens during particularly bad jokes rather than particularly good ones, as the embarassment as you realise just how poor your joke is triggers a nervous laugh

people who lumble tend to find humour in everything they see (a mobile library, small children, a lamppost etc.) and so are quite often heard lumbling their way through some half-assed joke that they really didn't think through